This invention relates to the field of styrene manufacture and more particularly discloses methods and apparatus, including reactor vessels, for the dehydrogenation of ethylbenzene into styrene monomer.
It is well known in the art of styrene manufacture to react ethylbenzene ("EB") over a dehydrogenation catalyst such as iron oxide under elevated temperatures in the range of 1000-1230.degree. F. and at a pressure of about 4 to 20 PSIA in order to strip hydrogen from the ethyl-radical on the benzene ring to form the styrene molecule. This might normally be done in a series of radial adiabatic styrene reactors which are commonly termed EB dehydro reactors. The dehydro reactors generally are elongated, cylindrical, vertical structures of a very large size, ranging in diameter from about five to about thirty feet or more, and in length from about ten feet to about one hundred feet or more. The normal construction for such a reactor allows for input of the ethylbenzene gas at an inlet located in the center of the vertical reactor, whereupon the gas is flowed radially outward through an annular area, passing through an annular porous catalyst bed of iron oxide or other suitable dehydro catalyst, and then passing through an outer annular area to exit the reactor shell. Since the flow of ethylbenzene across the catalyst bed is in a radial direction, these reactors are sometimes identified as "radial" reactors.
It is currently believed by those skilled in the art of styrene manufacture that the optimum arrangement of multiple, radial bed, EB reactors with typical dehydro catalyst beds is to utilize three or more radial adiabatic reactors arranged in serial-flow orientation, with reheat means between the reactors to add heat to the endothermic reaction. Each reactor may have a different selectivity catalyst from the catalyst of the other reactors. "Selectivity" in this instance is considered by one skilled in the art to mean the ability of the catalyst to selectively produce higher levels of the desirable styrene and lower levels of the undesirable toluene and benzene. "Activity" is considered to be the ability of the catalyst to convert a certain percentage of ethylbenzene to aromatics for each pass of feedstock over the catalyst. An example of the conventional radial reactor referred to above is that found in U.S. Pat. No. 5,358,698 to Butler, et al.
Because of the adiabatic design of conventional EB reactors and the endothermic nature of the dehydrogenation reaction, conventional EB processes require the addition of heat to the process to maintain the dehydrogenation reaction. This, in turn, necessitates the use of multiple reactors in order to provide opportunity to add heat during the process, which is accomplished by utilizing heaters or "superheaters" located between each of the serial reactors. This is also one reason that different catalysts are used in each of the serial reactors, with catalyst selectivity varying between the several reactors. Due to the endothermic nature of the EB reaction in the radial reactors, the liquid hourly space velocity (LHSV) through the system is severely limited. The EB feed must be flowed through the reactors slowly enough to allow dehydrogenation to be substantially completed, which is slowed by the absorption of heat in the reaction.
Therefore, a need has been felt for a process of dehydrogenating ethylbenzene which does not require large multiple reactors, heaters, heat exchangers, or multiple catalysts, and which is not limited by low LHSVs.